1. Field of the Invention
The invention relates to cranes having an elongated frame movable above the ground along a line perpendicular to the frame axis, a trolley movable along the frame, and a hoist on the trolley for causing vertical movement of a load engaging mechanism. More particularly, the invention relates to portal cranes.
2. Reference to Prior Art
Portal cranes are typically employed to handle material or objects stored in an elongated area several hundred or thousand feet in length, such as a log yard, pipe yard, or other material handling area. A portal crane has an elongated frame and typically moves along the elongated area or runway on a pair of spaced, parallel rails extending perpendicular to the longitudinal axis of the frame. A trolley is supported for movement along the frame, and a hoist on the trolley is connected to a load engaging mechanism such as a grapple. The trolley, hoist and grapple are operable to engage a load of objects, such as logs, and the hoist is operable to raise the load to a vertical level above the objects stored in the area, thereby permitting the loaded crane to move along the rails and runway. After the crane reaches an appropriate unloading area, such as the log receiving area of a paper mill or the truck loading area of a pipe yard, the load is deposited. After depositing the load, the crane is again moved along the rails and runway to pick up another load for delivery to the unloading area.